As lovely as 3x20TB RAID sounds, RAID on btrfs is never a particularly good idea. https://wiki.archlinux.org/title/btrfs#Multi-device_file_sys...
RAID 1 or 0 is just as a good idea as ever.
The 3x20TB btrfs setup I mentioned is configured with RAID1C3 for metadata, and RAID1 for data, and works just fine with even or odd number of drives.
It's funny how people assumed RAID5 when they saw 3 drives.
I switched to this after years of running on ZFS, and for my workloads btrfs is faster on Linux (not to mention the licensing/packaging mess).
I do something like this, https://kb.synology.com/en-us/DSM/tutorial/What_was_the_RAID...
Basically, mdadm, then on top, format with BTRFS
OpenZFS works fine, but it can be tricky to install due to the license thing, not all distributions support it in their installer.
Bcachefs is said to be the up and coming contender for both, but it was merged to the mainline kernel in January, so probably at the moment it is tricky to install as well. I would think many of this year's distribution releases will make it available.
For all the talk about the raid5/6 issues on btrfs, people don't seem motivated enough to actually spend time fixing this. It's almost as if mdadm was enough and there wasn't that much drive to make it happen.
> And what is the next fs after zfs and btrfs?